Most of the fatty acids bind to albumin in the bloodstream to move, but a small amount of free fatty acids (FFA) may be present in blood or tissue. As reported by A. Laws et al., (Arteriosclerosis, Thrombosis, and Vascular Biology), this fatty acid particle (FFA particle) induces a specific biological reaction in a body, but mechanism thereof was not almost known. The reason is that it is difficult to artificially prepare a pure FFA particle in vitro, and a size of the particle may be unstable due to spontaneous agglomeration of the FFA particles. Therefore, in order to conduct various researches into diseases caused by the FFA in vitro, a technology of preparing a pure free fatty acid nanoparticle dispersion solution having stable dispersibility in a water-based dispersive medium has been required.